This invention relates to a valve for an anaesthetic rebreathing system and is a development on the valve described and claimed in RSA Patent No. 78/4626--Manicom, Schoonbee--which describes such a valve.
The terms "anaesthetic system" and "anaesthetic rebreathing system" are used, in this specification, to denote the connecting apparatus between the anaesthetic machine and the face mask or endotracheal tube which connects to a patient, except where the context indicates otherwise.
A simple anaesthetic breathing system comprises two lengths of flexible tubing of suitable dimensions which connect the patient's airway to an anaesthetic gas supply machine. The two lengths of flexible tubing constitute the inspiratory and the expiratory limbs of a breathing circuit and converge at the patient's airway so that fresh gas flows from the anaesthetic gas supply machine, through the inspiratory limb of the circuit, to the patient's airway. Exhaled gas flows out through the expiratory limb of the circuit and is vented at the end of the expiratory limb. In a simple anaesthetic breathing system therefore, the flow of gas remains unidirectional.
In anaesthetic rebreathing systems, unlike simple anaesthetic breathing systems, a compliant reservoir bag is placed in the circuit and this provides for intermittent reversal of gas flow in whichever of the inspiratory or expiratory limbs contains the reservoir bag. When the inspiratory limb contains the reservoir bag the system is classified as a Mapleson A system and when the expiratory limb contains the reservoir bag the system is classified as a Mapleson D system.
A Mapleson A system is desirable when a patient breathes spontaneously, without being aided by manipulation of the reservoir bag, and such a system allows a low level of fresh gas flow to be used which prevents the wastage of costly anaesthetic gases. A Mapleson D system is desirable when the breathing of a patient is mechanically or manually controlled, for example, by compression of the reservoir bag. The rebreathing of exhaled gases inter alia provides useful humidification in the patient's airway.
It is important that in a particular situation the mode of breathing, namely spontaneous breathing or controlled breathing, is appropriate. If the inappropriate mode of breathing is used, at least twice the fresh gas flow would be required to prevent retention of carbon dioxide produced by the patient. This would be wasteful.